Lithographic printing plate precursors mainly employed in the field of small-scale commercial printing include (1) a direct drawing type printing plate precursor comprising a water-resistant support having provided thereon a hydrophilic image-receiving layer, (2) a printing plate precursor comprising a water-resistant support having provided thereon an oleophilic image-receiving layer containing zinc oxide, which is subjected to desensitizing treatment with a desensitizing solution after image formation to render the non-image area hydrophilic, thereby providing a printing plate, (3) a printing plate precursor of an electrophotographic light-sensitive material comprising a water-resistant support having provided thereon a photoconductive layer containing photoconductive zinc oxide, which is subjected to desensitizing treatment with a desensitizing solution after image formation to render the non-image area hydrophilic, thereby providing a printing plate, (4) a printing plate precursor of silver halide photographic material comprising a water-resistant support having provided thereon a silver halide emulsion layer, (5) a presensitized printing (PS) plate comprising an aluminum support having a hydrophilic surface having provided thereon a layer capable of forming a resin image upon exposure to an ultraviolet ray, and (6) a water-less presensitized printing (PS) plate comprising an aluminum support having provided thereon a photosensitive layer and a silicone rubber layer.
However, these printing plate precursors have problems, respectively. For instance, although printing plate precursors of type (1) are simple, they may not reach the satisfactory level in view of image qualities and background stains in the non-image areas required to prints, and printing durability (press-life). On the other hand, wet processing such as desensitizing treatment or sensitizing treatment is required in case of using printing plate precursors of type (2), (3) and (4), and treatment with an aqueous alkaline solution or an aqueous organic solvent solution is indispensable in case of using printing plate precursors (5) and (6) in order to form printing plates so that increased cost due to employing complicated and large-sized machines for the treatment and undesirable influence on environment caused by waste materials such as exhausted treating solutions are accompanied.
Recently, in the field of printing, computerization of plate-making steps has rapidly proceeded, and a plate-making system in which layout of letters, images and the like is determined on a computer and the information is directly output from an output device to a printing plate precursor draws attention.
Specifically, it is possible to directly produce a printing plate from digital data by exposure with a laser beam without using intermediate films and conventional optical printing methods. As the result, lithographic printing plate precursors which do not need wet processing such as desensitizing treatment or sensitizing treatment, development processing with an aqueous alkaline solution or an aqueous organic solvent solution and baking treatment have been proposed.
Examples thereof include a printing plate precursor having a heat-sensitive layer comprising a polymer containing heat-decomposable carboxylic ester groups associated with a compound capable of converting light to heat, which forms a printing plate by exposure with a heat laser to decompose the ester groups to generate carboxy groups, thereby rendering its surface hydrophilic, without wet processing, as described in EP-652,483, a printing plate precursor having a hydrophobic resin surface containing a compound capable of converting light to heat, which forms a printing plate by sulfonating to render its whole surface hydrophilic and exposing with a heat laser to remove the sulfonic groups by heating, thereby rendering its surface hydrophobic to form an image as described in JP-A-60-132760 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a printing plate precursor having a photosensitive layer containing a electroconductive polymer, which forms a printing plate by scanning an electrochemical or electric signal, thereby converting hydrophilic-hydrophobic property of its surface as described in EP-279,066, and a printing plate precursor comprising zirconia ceramic, which forms a printing plate by exposing its surface with a heat laser to convert oxidation state of the oxide, thereby changing its surface property from hydrophilic to hydrophobic as described in EP-769,372.
However, these lithographic printing plate precursors have one or more disadvantages which restrict practical use.
For example, difference in a property between an oleophilic image portion and a hydrophilic non-image portion is not large enough in many cases and fatal defect in that background stains occur in prints or printing ink dose not sufficiently adhere on the image portion is accompanied. Also, dampening water used for printing is restricted. Further, materials contained in the printing plate precursors may cause problems due to their poor storage stability.